The present invention relates generally to structured packings, and more particularly, to structured packing elements and applications to method of cryogenic separation.
Structured packings have found widespread use in a variety of distillations including those involved in the separation of air into its component parts. Distillations are conducted within distillation columns filled with mass transfer elements to bring ascending vapor phases into intimate contact with descending liquid phases of mixtures to be separated. As the ascending phase rises and contacts the descending liquid phase, it becomes evermore enriched in the more volatile components of the mixture to be separated. At the same time, the descending liquid phase becomes ever more concentrated in the less volatile components of the mixture to be separated. In such fashion, systems of distillation columns can be used to separate various mixture components. For instance, in case of air separation, nitrogen is separated from oxygen in a double distillation column unit. Argon is then separated from oxygen in an argon column that is attached to a lower pressure column of such a double distillation column unit.
Structured packings are widely used as mass transfer elements within distillation columns due to their low pressure drop characteristics. Structured packings generally include a series of structured packing elements that are made up of a number of corrugated sheets of material, in which the sheets are placed in a side by side relationship with the corrugations of adjacent sheets criss-crossing one another. In use, the liquid phase of the mixture to be separated is distributed to the top of the packing and spreads out throughout the packing as a descending film. The vapor phase of such a mixture rises through the corrugations contacting the liquid film as it descends.
There have been many attempts in the prior art to increase the efficiency of structured packings, that is, to decrease the height of packing equal to a theoretical plate (HETP). Obviously, the lower the height, the more efficient the packing. At the same time, a structured packing with a low HETP inherently has an increased pressure drop over less efficient packings. One such structured packing is disclosed in U.S. Pat. No. 4,597,916 in which the corrugated sheets are separated from one another by flat, perforated sheets that extend throughout the packing. It is believed that the flat perforated sheets of this prior packing increase efficiency by both providing additional interfacial area for vapor-liquid contact and by increasing turbulence in the vapor flow and therefore the degree of mixing between vapor and liquid phases. Other approaches to improving the performance of structured packings have also been disclosed in U.S. Pat. No. 5,632,934 and EP 858,366B1, which involve modifying the configurations of corrugations close to the interfaces between adjacent structured packing elements. It is believed that such modifications lead to improved performance by reducing the pressure drop between adjacent packing elements. However, there is still an ongoing need for alternative designs of structured packings for improving capacity without significant sacrifice in the separation efficiency, and vice versa.
The present invention provides generally a structured packing with improved capacity without significant loss of separation efficiency and a method of cryogenic separation using the structured packing. In one embodiment, a structured packing of the present invention contains packing elements comprising an array of vertically oriented corrugated sheets, and one or more planar members located between at least one pair of adjacent corrugated sheets. At least some of the corrugated sheets have corrugations in the middle portion with angles of inclination from horizontal that are smaller than angles of inclination in at least one of the top and bottom portions of the corrugated sheets. The one or more planar members and the corrugated sheets are positioned such that at least one outermost horizontal edge of each planar member is proximal to a horizontal edge of the adjacent corrugated sheets. Another aspect of the invention provides a method of cryogenic separation using such a structured packing.
Other embodiments relate to structured packing elements comprising corrugated sheets that have differences in configurations between a middle portion and at least a top or a bottom portion of the corrugated sheet, and one or more planar members positioned between at least one pair of adjacent corrugated sheets. Corrugations in the middle portion of each corrugated sheet are characterized by a corrugation height and a corrugation width. The planar members are positioned such that an outermost horizontal edge of each planar member is proximal to a horizontal edge of the adjacent corrugated sheets, and each planar member has a length that is equal to at least two times the corrugation width of adjacent corrugated sheets.
One specific embodiment relates to a structured packing element with planar members used in conjunction with corrugated sheets having corrugations whose angles of inclination at the middle portion of the corrugated sheet are less than 90 degrees, while those at the top and bottom portions increase progressively to about 90 degrees. Another specific embodiment relates to a structured packing element with planar members used in conjunction with corrugated sheets having rectilinear corrugations at the middle portions with angles of inclination less than 90xc2x0 (typically from about 30xc2x0 to about 60xc2x0), while those at the bottom portions have angles of inclination about 90xc2x0. In both embodiments, the two planar members located between each pair of adjacent corrugated sheets both have a length that is equal to at least two times the corrugation width of an adjacent corrugated sheet.